A Fresnel optical element includes a plurality of Fresnel prisms arranged on a base and in a sawtooth shape, each of the plurality of Fresnel prisms having a refractive surface for refracting a light ray emitted from a light emitting body, such as a projector, and a reflective surface for reflecting the light ray refracted by the refractive surface.
In each of the plurality of Fresnel prisms, a part of an incident light ray cannot pass through the refractive surface in which the refractive index varies between media sandwiching the surface and the light emergence surface of the base in which the refractive index varies between media sandwiching the surface according to the Fresnel law of reflection. About 5% of the incident light ray does not pass through the refractive surface and the light emergence surface of the base depending on the refractive indices at each of these surfaces.
For example, a light ray reflected by the refractive surface without passing through it and a light ray reflected by the light emergence surface of the base (referred to as unnecessary light from here on) among the rays of light emitted from the light emitting body may be made to propagate toward different places while being repeatedly refracted and reflected by the plurality of Fresnel prisms, and may emerge from positions of the Fresnel optical element which are different from those from which rays of light (referred to as signal light) reflected by the reflective surfaces of the plurality of Fresnel prisms are made to emerge.
Since a ghost image which is identical to an image provided by the signal light is recognized by the viewer when such unnecessary rays of light are made to emerge toward the viewer's line of sight, the contrast ratio of the projection display apparatus degrades remarkably.
Then, by adjusting the angle of the reflective surface of each of the plurality of Fresnel prisms so that it satisfies appropriate requirements, the related art Fresnel optical element carries out control processing so that the angle of the light ray reflected by the light emergence surface of the base which results in the generation of the ghost image is larger than the angles of the rays of light emitted from the light emitting body to prevent unnecessary rays of light from emerging toward the viewer's line of sight (refer to patent reference 1, for example).
It is known that such the related art Fresnel optical element can be replicately molded by pouring a resin material to a metallic mold which is machined using a large-sized lathe.
Patent reference 1: JP, 2002-196413,A (see paragraph numbers [0011] to [0019] and FIG. 1)
A problem with the related art Fresnel optical element constructed as mentioned above is that although rays of light reflected by the light emergence surface of the base can be prevented from emerging, as unnecessary rays of light, toward the viewer's line of sight, rays of light reflected by the refractive surfaces of the plurality of Fresnel prisms of the Fresnel optical element without passing through them are made to emerge, as unnecessary rays of light, toward the viewer's line of sight.
The present invention is made in order to solve the above-mentioned problems, and it is therefore an object of the present invention to provide a Fresnel optical element that can prevent unnecessary rays of light, such as rays of light which are reflected by the refractive surfaces of the Fresnel optical element without passing through them, from emerging toward the viewer's line of sight.
It is another object of the present invention to provide a projection display apparatus that can prevent display of any ghost image.